In LTE (Long Term Evolution) system, the orthogonal frequency division multiple access (OFDMA) and single carrier FDMA (SC-FDMA) technologies are adopted in the downlink and uplink respectively. In LTE-advanced system, these multiple access technologies are still the candidate schemes. The inter-cell interference is the main limitation factor for capacity in such systems with low frequency reuse factor. If a mobile station is located at the cell boundary, interference from neighboring cells is a crucial factor leading to severe performance degradation (especially throughput and coverage).
Static and semi-static coordination technologies are the usual interference coordination technologies in LTE proposals. These inter-cell interference coordination technologies propose restrictions for a coordinating way between cells. For example, these restrictions may be in the form of restrictions to time/frequency resources available to the resource manager, or restrictions on the transmit power applicable to may certain time/frequency resources.
A static solution is proposed in reference [1]. FIG. 1 shows the frequency reuse scheme of this proposal. As shown in FIG. 1, seven cells are shown, cells 2 to 7 are adjacent cells of cell 1. In order to decrease or eliminate most of the interference, on the edge of cells, the frequencies should be orthogonal between neighbor cells, and in order to increase the utilization ratio of frequency resource, the frequency reuse factor should be increased as much as possible. For example, on the edge of each cell, cell 1 uses frequency 1, cell 2, 4 and 6 use the same frequency 2, cell 3, 5 and 7 use the same frequency 3. Cell-edge users transmit/receive with the full power. Cell-center users use the whole spectrum with restricted power to transmit/receive, and hence there will not be too much inter-cell interference (ICI) even the same frequency is used. However, there are two obvious disadvantages: (1) the frequency utilization rate of cell-edge users is only ⅓; (2) the solution may not be fulfilled optimally if the traffic load of the service changes.
Another static solution named as network power planning is proposed in reference [2], the whole frequency band is divided into several subsets with corresponding power, for example, 7 to 9 subsets. The frequency utilization rate of cell-edge users is 3/7 according to this proposal.
A semi-static scheme is proposed in reference [3]: the entire frequency band is divided into N sub bands, then X sub bands are used for cell-edge users and N−3X sub bands are used for cell-center users. The X sub bands used for cell-edge users are orthogonal in neighbor cells, while the N−3X sub bands used for cell-center users are the same in all cells. Only a part of spectrum is used in the inner cell and cell edge respectively in this scheme.